The present invention relates to multilayer mirrors for extreme ultraviolet radiation. More particularly, the invention relates to the use of such mirrors in lithographic projection apparatus comprising:
an illumination system for supplying a projection beam of radiation;
a first object table provided with a mask holder for holding a mask;
a second object table provided with a substrate holder for holding a substrate; and
a projection system for imaging an irradiated portion of the mask onto a target portion of the substrate.
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, catadioptric systems, and charged particle optics, for example. The illumination system may also include elements operating according to any of these principles for directing, shaping or controlling the projection beam, and such elements may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. In addition, the first and second object tables may be referred to as the xe2x80x9cmask tablexe2x80x9d and the xe2x80x9csubstrate tablexe2x80x9d, respectively.
In the present document, the invention is described using a reference system of orthogonal X, Y and Z directions and rotation about an axis parallel to the I direction is denoted Ri. Further, unless the context otherwise requires, the term xe2x80x9cverticalxe2x80x9d (Z) used herein is intended to refer to the direction normal to the substrate or mask surface or parallel to the optical axis of an optical system, rather than implying any particular orientation of the apparatus. Similarly, the term xe2x80x9chorizontalxe2x80x9d refers to a direction parallel to the substrate or mask surface or perpendicular to the optical axis, and thus normal to the xe2x80x9cverticalxe2x80x9d direction.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto an exposure area (die) on a substrate (silicon wafer) which has been coated with a layer of photosensitive material (resist). In general, a single wafer will contain a whole network of adjacent dies which are successively irradiated via the reticle, one at a time. In one type of lithographic projection apparatus, each die is irradiated by exposing the entire reticle pattern onto the die in one go; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94which is commonly referred to as a step-and-scan apparatusxe2x80x94each die is irradiated by progressively scanning the reticle pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the wafer table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally  less than 1), the speed V at which the wafer table is scanned will be a factor M times that at which the reticle table is scanned. More information with regard to lithographic devices as here described can be gleaned from International Patent Application WO97/33205, for example.
Until very recently, lithographic apparatus contained a single mask table and a single substrate table. However, machines are now becoming available in which there are at least two independently moveable substrate tables; see, for example, the multi-stage apparatus described in International Patent Applications WO98/28665 and WO98/40791. The basic operating principle behind such multi-stage apparatus is that, while a first substrate table is at the exposure position underneath the projection system for exposure of a first substrate located on that table, a second substrate table can run to a loading position, discharge a previously exposed substrate, pick up a new substrate, perform some initial measurements on the new substrate and then stand ready to transfer the new substrate to the exposure position underneath the projection system as soon as exposure of the first substrate is completed; the cycle then repeats. In this manner it is possible to increase substantially the machine throughput, which in turn improves the cost of ownership of the machine. It should be understood that the same principle could be used with just one substrate table which is moved between exposure and measurement positions.
In a lithographic apparatus the size of features that can be imaged onto the wafer is limited by the wavelength of the projection radiation. To produce integrated circuits with a higher density of devices, and hence higher operating speeds, it is desirable to be able to image smaller features. Whilst most current lithographic projection apparatus employ ultraviolet light generated by mercury lamps or excimer lasers, it has been proposed to use shorter wavelength radiation of around 13 nm. Such radiation is termed extreme ultraviolet (EUV) or soft x-ray and possible sources include laser plasma sources or synchrotron radiation from electron storage rings. An outline design of a lithographic projection apparatus using synchrotron radiation is described in xe2x80x9cSynchrotron radiation sources and condensers for projection x-ray lithographyxe2x80x9d, J B Murphy et al, Applied Optics Vol. 32 No. 24 pp 6920-6929 (1993).
In the EUV spectral region high reflectivity mirrors, apart from grazing incidence mirrors, must necessarily be multilayered thin film designs. The predominant designs are composed of distributed Bragg reflectors resembling quarter wavelength stacks with constant film thicknesses throughout. For the 11-16 nm wavelength region two designs predominate: Mo/Be for the 11.3 nm window consisting typically of 80 periods and the Mo/Si system for the 13.4 nm window of 40-50 periods, both with a partition ratio xcex93=0.4, where xcex93=dMo/(dMo+dSi(Be)). In general, the partition ratio is defined as the ratio of the thickness of the material having the higher extinction coefficient, k, to the total thickness of the two layers. These designs yield maximum theoretical reflectivities of Rxcx9c0.78 for the Mo/Be stack, and Rxcx9c0.74 for the Mo/Si stack while taking into account a highly absorbing xcx9c2 nm native oxide on the surface Si layer. These reflectivity values (which are amongst the best for multilayer reflectors in the EUV region), whilst adequate for optical systems with a few reflectors, will dramatically diminish the output optical intensity to 6-10% of that directly before the first mirror in, for example, a nine-mirror system. The significance of nine mirrors is that this is the number envisaged for an EUV lithographic system; two in the illumination optics, six in the imaging optics plus the reflecting reticle. It is therefore evident that even a xe2x80x9csmallxe2x80x9d increase of 1-2% in the peak reflectivity of a single mirror will yield a significant light throughput enhancement of the optical system.
It is an object of the present invention to provide multilayer mirrors for extreme ultraviolet radiation (EUV) that have higher reflectivities at desired wavelengths.
According to the present invention, this and other objects are achieved in a reflector for reflecting radiation in a desired wavelength range, the reflector comprising a stack of alternating layers of a first and a second material, said first material having a lower real refractive index in said desired wavelength range than said second material, characterised by:
at least one layer of a third material interposed in said stack, said third material being selected from the group comprising Rb, RbCl, RbBr, Sr, Y, Zr, Ru, Rh, Tc, Pd, Nb and Be as well as alloys and compounds of such materials.
In preferred embodiments of the invention, a layer of said third material is interposed between each pair of layers of said first and second materials, and optionally at least one layer of a fourth material may be interposed in said stack, said fourth material being selected from the group comprising Rb, RbCl, RbBr, Sr, Y, Zr, Ru, Rh, Tc, Pd, Nb and Be as well as alloys and compounds of such materials.
The present invention also provides a reflector for reflecting radiation in a desired wavelength range, the reflector comprising a stack of alternating layers of a first and a second material, said first material having a lower real refractive index in said desired wavelength range than said second material, characterised in that:
the layer thicknesses of said first and second materials vary through the stack.
The layer thicknesses are preferably determined by a global or needle optimisation technique.
The invention further provides a reflector for reflecting radiation in a desired wavelength range, the reflector comprising a stack of alternating layers of a first and a second material, said first material having a lower real refractive index in said desired wavelength range than said second material, characterised in that:
said second material is selected from the group comprising P, Sr, Rb and the lanthanides, especially La, Ce, Pr and Eu, as well as compounds or alloys thereof.
Still further, the present invention provides a reflector for reflecting radiation in a desired wavelength range, the reflector comprising a stack of alternating layers of a first and a second material, said first material having a lower real refractive index in said desired wavelength range than said second material, characterised in that:
said first material is selected from the group comprising Ru and Rh as well as alloys and compounds thereof.
Reflectors according to the invention may have a capping layer of a relatively inert material, which is preferably selected from the group comprising diamond-like carbon (C), silicon carbide (SiC), boron nitride (BN), silicon nitride (Si3N4), B, Ru and Rh and preferably has a thickness in the range of from 0.5 to 3 nm, preferably in the range of from 1 to 2 nm.
A second aspect of the invention provides a lithographic projection apparatus comprising:
an illumination system for supplying a projection beam of radiation;
a first object table provided with a mask holder for holding a mask;
a second object table provided with a substrate holder for holding a substrate; and
a projection system for imaging an irradiated portion of the mask onto a target portion of the substrate; characterised in that:
at least one of said illumination system and said projection system includes a reflector as described above.
A third aspect of the invention provides a method of manufacturing a reflector for reflecting radiation in a desired wavelength, the reflector comprising a stack of alternating layers of a first and a second material, wherein the first material has a lower real refractive index than the second material, the method comprising the steps of:
determining appropriate layer thicknesses using a numerical iterative optimisation process; and
manufacturing the reflector with layer thicknesses substantially as determined in the determining step.
In preferred embodiments of the third aspect of the invention, the iterative process comprises:
establishing a model of a reflector having specified materials for the first and second materials and specified initial thicknesses for the layers;
varying the thicknesses of one or more layers of the stack and calculating the reflectivity of the resultant stack; and
repeating the varying and calculating steps until a specified criterion is reached.
Optionally in the establishing step, at least one layer of at least one third material is also included in the stack, the thickness of said layer of third material being varied in at least one iteration of the varying step.
With the invention, reflectivity enhancements in the standard Mo/Be and Mo/Si stacks can be achieved by one or more of:
(1) incorporating additional materials within the basic stack,
(2) replacing one of the components of the standard stack by one with more favourable optical constants,
(3) utilising global optimisation routines to vary the partition ratio or the individual film thicknesses within the stack for optimum peak reflectivity, and
(4) selecting certain relatively inert materials as capping layers to avoid the formation of highly absorbing surface oxide films.
The various materials usable in the invention, in addition to molybdenum (Mo), silicon (Si) and beryllium (Be), are derived mainly from period 5 of the periodic table of elements and include: rubidium (Rb), rubidium chloride (RbCl), rubidium bromide (RbBr), strontium (Sr), yttrium (Y), zirconium (Zr), ruthenium (Ru), rhodium (Rh), palladium (Pd), technetium (Tc), phosphorous (P), boron (B) and niobium (Nb). Alloys and compounds of these materials may also be used.
Other materials usable in the present invention are the lanthanides, from lanthanum to lutetium, but especially: lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm) and europium (Eu). These and the other lanthanides mentioned above may be used with: phosphorous (P), niobium (Nb) and antimony (Sb).
In addition to the pure elements, compounds of the above materials may be used, especially the borides, carbides, nitrides, phosphides, halides (e.g. CsI). Alloys of the metals noted, and including the Group IIA elements, may also be employed in the invention.
Still further materials useable in the invention are low density porous materials such as silica, titania and alumina aerogels; nano-porous silicon, meso-porous silicon, nanoclusters of silicon and other semiconductors
In embodiments of the invention, any or all of the layers may have other materials or elements implanted or diffused into them, e.g. to effect any desired alterations to their optical, chemical or mechanical properties.
With the invention it is possible to provide a reflector optimised to a particular radiation source, especially in the 8 to 16 nm wavelength range.
A fourth aspect of the invention provides a device manufacturing method comprising the steps of:
providing a substrate which is at least partially covered by a layer of energy-sensitive material;
providing a mask containing a pattern;
using a projection beam of radiation to project an image of at least part of the mask pattern onto a target area of the layer of energy-sensitive material; characterised in that:
said projection beam of radiation is supplied or projected using an illumnation or projection system including a reflector according to the first aspect of the invention.
In a manufacturing process using a lithographic projection apparatus according to the invention a pattern in a mask is imaged onto a substrate which is at least partially covered by a layer of energy-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping) metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget areaxe2x80x9d, respectively.